VK112 Bias Pot Mod

[CharlieP]

I did a bias pot install today with a 15k ohm series resistor. I have not only found improvement in tone but the reverb works better and even the texture switch. Also the lead channel sounds better.

[CharlieP]

This is a follow up. I have had the new bias pot for 11 days of constant use. I have observed no problems and much benefit. This amp has really come alive. I am not sorry that I have done this mod. One item I would change if I had to do it again is, Instead of a single 15k resistor, I would use one 6.8k on each connection to the pot. This would mainly make it easier to solder with one less joint as opposed to the method I used. Two 6.8k = 13.6k, close enough for a buffer. I also think my choice of pots was the correct way to go especially for mounting and ease of adjustment. I can adjust without pulling the chassis. It was well worth while. It is the best thing I have done with the amp. Some people talk of benefits in changing the OT. I do not think I really need to do that now. It sounds great to me with the stock OT.

[Enzo]

It looks great in the photos.

I don;t follow how splitting the resistance into two parts instead of one simplifies the wiring though. Unless I don;t understand, as it is you have two ends of a resistor and two ends of a wire to solder - four connections. With two resistors you'd have two ends of two resistors to solder - four connections.

[CharlieP]

Originally Posted by Enzo

It looks great in the photos.

I don;t follow how splitting the resistance into two parts instead of one simplifies the wiring though. Unless I don;t understand, as it is you have two ends of a resistor and two ends of a wire to solder - four connections. With two resistors you'd have two ends of two resistors to solder - four connections.

My jumper is 2 pieces of leftover resistor leads and a possible fail due to vibration and solder cracking. Whereas using a whole resistor or a whole wire would be a better option. It is soldered mid point.

[pburgbahamut]

Hey! I was wondering if this would work on a Valveking 212. I looked at the Schematics and they seemed identical. Would I need to change the values on the resistor?

[CharlieP]

Originally Posted by pburgbahamut

Hey! I was wondering if this would work on a Valveking 212. I looked at the Schematics and they seemed identical. Would I need to change the values on the resistor?

The same 15k will work. Just preset the resistance of both the 15k in series with the pot to 39k as a good starting point to do bias. The resistor you are removing is 39k. This gets you in the factory ballpark to start.

[pburgbahamut]

Hey,

I just found some time to do this thing and I'm gathering the supplies. Do you think I can just use a 50k Trim pot and skip the resistor? I think it would make things easier.

[Enzo]

I don;t know that one resistor is enough to make things complex, especially if you are using is as a piece of wire. You have to cut and strip a piece of wire and install it or you have to bend the legs of a resistor and install it.

The whole point of the added resistor is so that the pot can be turned all the way down without killing the bias completely. Furthermore, with just a pot you wind up using only one end of it in its adjustment range.. Like on an amp that was so loud you could only use 0-2 on the volume control. By adding the resistor - and I would recommend a 25k pot instead of 50k myself - then we can spread the useful range of adjustment over a larger portion of the pot travel.

[CharlieP]

I have no problem with the pot I selected. It was real easy to mount and have external access to do bias. It is easy to adjust. As Enzo states, it is not recommended to skip the resistor just because you are anxious. Do the job right and you will have something that has a bit of safety buffer for your amp. Please do not shortcut, it is to your benefit. These parts are nickel and dime items. Will not break your piggy bank. It is worth the wait to order the right parts. If you can find a bias pot similar that is 25k, sure get it. Not necessary tho in my opinion. Take your time and make sure of all solder joints flow properly. If solder rounds up in a ball it is a bad joint. Make sure chassis and board is free of chips from drilling hole. We do not want a short circuit from them. Be sure your soldered leads do not touch the chassis under the solder terminals.
Make sure you understand how to discharge the caps properly and check voltages to monitor them. This is your responsibility and I do not assume any. Your safety is your responsibility. A good start is to shut the main power switch off only and unplug cord. You do not need to have power to the chassis while doing the installation. It is suggested you have a Bias Rite or similar bias probe. Much easier and IMO safer.

[CharlieP]

See these instructions I posted in UG.
ultimate-guitar-valveking - Amp Mods - Electronics
Read down the page to find them. Step by step guide.

[pburgbahamut]

Oh OK. Yea I get the point about the resistor. I was kinda looking at putting one of these in:

3059P-1-503LF

I believe this multi-turn trim pot has a range of approx. 22 turns. So in a 50k that would mean about 1k per turn.

I was also gonna skip the drilling the whole part. I wouldn't really mind taking the chassis out every time I changed tubes to rebias.

[Enzo]

You can use any style of pot you like. I like to have them available from the outside myself too.

Charlie, I don;t see a problem with the 50k. If I were to start from scratch I'd order a pot that fit my needs. I think you chose 50k because it came with the locking shaft, is that not correct? I have a number of the locking assemblies that are separate. You can slide them over any regular shaft with 3/8 bushing and thread the thing on top of the mounting nut. The the locking nut threads over that. it is exactly what you have except it is not built into the pot.

My rationale for 25k was that the whole thing started at 39k. The reason to mod the amp was to get the bias hotter than stock. SO and increase in that 39k would make bias colder - a direction we would not likely need to go. SO with a 15k safety and a 25k pot, 40k is as close to 39k as we get with common parts. The mod then goes from stock cold to hotter levels. and it spreads it out over a wider portion of pot rotation.

I didn't mean to imply 50k was in any way a problem.

[CharlieP]

Originally Posted by pburgbahamut

Oh OK. Yea I get the point about the resistor. I was kinda looking at putting one of these in:

3059P-1-503LF

I believe this multi-turn trim pot has a range of approx. 22 turns. So in a 50k that would mean about 1k per turn.

I was also gonna skip the drilling the whole part. I wouldn't really mind taking the chassis out every time I changed tubes to rebias.

I find it inconvenient to take the chassis out to do a bias. I do not recommend it. It is just not the way I would do it.
Enzo, the lock is one of the reasons for that pot and ease of use once mounted. I already had a 25k pot and it just wasn't going to work for me.

[Enzo]

For future reference: (I only looked in Mouser, I am sure there are other resources.)

Shaft locks
http://www.keyelco.com/pdfs/p56.pdf

And just searching "shaft lock" I had no idea they had so many locking pots in so many shapes and styles. I associate shaft locks with old equipment, but I guess they are still in wide use.

[CharlieP]

pburgbahamut: What scares you about drilling a hole? It is just one well placed clearance hole for a ¼" threaded shaft. It is not a problem to do and really makes the outside adjustment work.

[pburgbahamut]

Well its not that I'm afraid to do it or anything. Its just that I take out the chassis for my Carvin X100b anyways to do a bias. So I'm kind of used to it.

[CharlieP]

Originally Posted by pburgbahamut

Well its not that I'm afraid to do it or anything. Its just that I take out the chassis for my Carvin X100b anyways to do a bias. So I'm kind of used to it.

It is just as easy to have the convenience of biasing without taking the chassis out. Also how to mount the pot is figured. Other style you will have to figure it out. The 50k Weber pot should work even better for your amp with the 39k resistor, allowing you more adjustment range. It really makes a tube change and bias quick and easy.

[CharlieP]

Just an update here! I acquired my second VK112 and did a mod to it. This time I used (2) 6.8K resistors. One in and one out in series path. Used the same Weber 50K Bias Pot as the first time. Using two resistors was just a convenience of attaching. Last time I used (1) 15K resistor and for the second path connection I had soldered (2) clipped resistor leads together to bridge the connection from board to pot. I felt that may create a future fail point with vibration.

[docz]

Did you have to take out the PCB to access the solder points? Or did you just top solder them?

DocZ

[CharlieP]

Originally Posted by docz

Did you have to take out the PCB to access the solder points? Or did you just top solder them?

DocZ

You have to flip the board to solder correctly. Cut the cable tie binding the standby switch to get slack enough and take the standby loose also. Gives you enough slack to flip the board and put a wood spacer under board to keep it from falling down in the chassis while you are working. There is also a small cable that can be unplugged from the power tube board. First time I did this mod I didn't unplug it the second one I did and it was a little easier.


Warning: Do not attempt this unless you fully understand how to discharge caps. High voltage and dangerous.